1. Field of the Invention
The present invention relates to gloss sensors for optically measuring the gloss of a moving surface, and, more particularly, to a gloss sensor used to measure the gloss of a moving fiber web in a paper machine.
2. Description of the Related Art
In the paper making industry where paper is being produced at a high rate from a paper making machine, for quality and feedback control the paper is scanned crosswise by a moving a head containing a number of sensors to determine parameters such as basis weight, moisture and gloss. The final value of gloss is a rather arbitrary number determined by standards in the paper making industry; namely, TAPPI standard T 480 om-90 which involves projecting onto the paper surface an incident beam of light at a particular angle, detecting the reflected beam and measuring its intensity. To calibrate the above TAPPI standard a polished black glass standard is used. Then an intermediate standard which is calibrated against that may be a polished ceramic tile. Some gloss sensors actually mechanically carry such a tile in a moving measuring head and lower the tile into the light beam to calibrate the instrument. This, of course, is mechanically complicated and there are some problems of environmental conditions such as heat, dirt and also accurate positioning.
Another gloss measuring technique uses a separate reference beam apart from the incident measuring beam. Here there are two separate light sources and light detectors. Moreover, the light source is a different type than the standard source defined by the above TAPPI standards. Thus, the correlation to the industry standard is suspect.
It is also known to use a gloss sensor which is calibrated using an oscillation angle light source which with the same lamp has a “reference” to a light detector. A reference beam is split from the main beam emitted from the light source and passes through a recess in the housing which is external to the gloss sensor. The reference beam then passes across the recess and through a detector window to re-enter the gloss sensor, then through a lens, and finally changes angles at a reflector to impinge upon the light detector. An example of such a gloss sensor is disclosed in PCT/US98/08805, which is assigned to the assignee of the present invention. This type of gloss sensor works well, but is still somewhat bulky in size due to the geometric layout of the various components. Moreover, the recess in the gloss sensor housing may allow accumulation of dirt or debris which can affect the accuracy of the gloss sensor.
What is needed in the art is a gloss sensor which is compact and has a high accuracy and reliability.